Change in the Air for Climate ControlPosted 4/14/2004
By Jim Taylor
Vehicle manufacturers are under increasing regulatory pressure. Emissions from air conditioning (A/C) system refrigerant leakage must be reduced to protect the environment. Recovery and recycling of refrigerants are key considerations. Moreover, huge changes loom on the European horizon as pending legislation moves to eliminate R134a by the year 2018. Since we live in a global marketplace, this decision will affect European, U.S. and Asian vehicle builders.
Against this industry background, the Society of Automotive Engineers (SAE) convened its fifth Alternate Refrigerant Systems Symposium last year in Scottsdale, Ariz. The event encourages presentation of research and the exchange of technical information between equipment manufacturers, vehicle builders, government officials and aftermarket suppliers.
The session opened with a review of the Summit held in Brussels, Belgium, in February 2003. Representatives of almost every international vehicle manufacturer and component supplier attended that event. Attendees agreed the HFC-134a used in automotive air conditioning is a contributor to global warming through the release of greenhouse gases.
There are many options for reducing these direct emissions, including enhanced (tighter) systems that leak less and/or use refrigerants with a lower global warming potential (GWP). Additional strategies may include discouraging the purchase of mobile air conditioning systems through higher prices or additional taxes on offending refrigerants.
Depending on the system type and refrigerant selected, there are several other areas of concern. Can emissions, such as from the tailpipe, be reduced during A/C use through better power plant and compressor management? As well, certain new A/C refrigerants will require changes to the service infrastructure, adding new shop equipment, more training, and possibly additional certification requirements for service technicians. Occupant risks associated with new refrigerants must be assessed and mitigated to ensure safety and reduce liability.
The European Connection
Dr. Matti Vainio, principal administrator for the European Commission's Environment Directorate General, presented the guidelines and legislation proposed for adoption within the European Community (EC). To put the problem in perspective, he began with a simple question: How long would you have to park your car to approximately balance the atmospheric damage done by the rupture of a hose on a contemporary HFC-134a system? Although the charge within a modern air conditioning system is relatively small, the environmental damage caused by a full- system release is roughly equal to six months of tailpipe emissions.
Additionally, a study by the European Commission revealed an average system leakage of approximately 51.8 grams/yr; with some offenders leaking as much as 70 g/yr. Surprisingly, the study found greater leakage in the first year of a vehicle's life than in the second or third. (Studies in North America found lesser leakage, quoting loss rates from 15-30 g/yr.) The Commission proposes to address these problems on cars and light trucks through a system of quotas, credits and limits that become stiffer over time and will eventually eliminate, in Europe, the use of HFC-134a. The solution was arrived at through the cooperation of the OEMs, component manufacturers, governments and other stakeholders. The rules will apply to anyone selling vehicles in Europe, regardless of where the vehicle is made. Although subject to some change as it goes through the legislative process, the proposal addresses many concerns by allowing the industry flexibility and a realistic time line to accomplish the required goals.
For each company selling vehicles in the EC, the number of European sales will be tallied for a period of years - possibly 2000 through 2003, but yet to be decided - and an average yearly sale number will be determined. This average becomes the "base number." Cars sold outside the EC do not count, but cars without air conditioning sold in the EC are included in the total. Beginning in 2008, HFC-134a production limits - quotas - go into effect, expressed as a percentage of the base year sales. The penalty for exceeding the quotas is proposed at 500 Euros (approximately $561 at present exchange rates) per vehicle.
For example, the 2008 quota is 80 percent of base year sales. Thus, a company with a base year average of one million vehicles sold could only produce 800,000 HFC-134a cars. By 2012, the quota is 0 percent - no cars may be manufactured with HFC-134a refrigerant.
Even before the quotas take effect, moves are under way to improve present conditions. On Jan. 1, 2005, HFC-134a systems sold in the EC must be certified to leak no more than 40 g/yr for single-evaporator systems or 50 g/yr for dual evaporator systems. 2005 will also see limitations on the sale of refrigerant in "small cans" as well as mandates for recovery and recycling of HFC-134a.
Vainio acknowledged that accomplishing the goal of eliminating a polluting chemical from Europe within 15 years would require full cooperation from the European, North American and Asian vehicle manufacturers.
He also touched on some of the open issues surrounding this legislation. As of now, there is no formal, industrywide procedure for certifying the leakage rate of an "enhanced" HFC-134a system. He called for an industrywide standard, possibly developed by SAE or ISO, so all cars undergo the same test prior to certification. Other issues include the need to support research and development of new refrigerant systems as well as the development of support equipment to install and service the systems. He noted the additional need to reduce tailpipe emissions resulting from A/C use and also to reduce the associated "fuel penalty" for A/C use.
The European Commission's proposed legislation would be sent to both the Council of Member States and the Parliament. It will become law after both governmental houses ratify it; agreement is anticipated by the end of 2004.
"Performance, not Prescription"
Dr. Stephen O. Anderson, director of Strategic Climate Projects for the Climate Protection Division of the U.S. Environmental Protection Agency (EPA), acknowledged remarkable cooperation between the European Commission, national governments worldwide, vehicle manufacturers, component suppliers and the scientific community in the pursuit of alternate refrigerants.
With the global effort to achieve minimal HFC emissions, the Brussels Summit showed both technical optimism and a firm grasp of reality. Presentations by technical experts allowed the European Commission to see the time lines necessary for development of the alternate refrigerant systems and to develop strategies allowing true competition. A key step in their deliberations was to adopt the policy of viewing proposed alternatives by their environmental acceptability, notably their potential as a greenhouse gas. The regulators in North America, Japan and other nations are mindful of the advantage of having complimentary regulations so vehicles can be profitably sold in global markets and comply with all requirements in effect.
Anderson called for the SAE to develop a simple procedure for testing and certifying systems as compliant with the European 2005 regulations. It would include component testing specifications and the details necessary for a globally recognized test method. In each manufacturing region, a third-party organization - not the vehicle builder - would certify that the components and systems are tested as prescribed and that they comply with the applicable rules. He also issued a call for international cooperation and collaboration on the availability of test results and the adjustment of testing procedures if necessary. The testing and certification requirements need to be developed quickly if enhanced systems are to be certified by third-party organizations for 2005 vehicles.
In the United States, the approach to climate protection is more voluntary than it is in Europe, and the EPA's intention is to move ahead with voluntary programs to satisfy obligations under various treaties. Using the phrase "Performance, not Prescription," he noted many manufacturers have already been working on improved components for HFC-134a systems. Thus, it should be possible to quickly develop a plan to satisfy requirements for an enhanced system.
Studies show taxes on refrigerants can encourage development of tighter systems and use of lower system charge volumes. During the phase-out of CFC-12 in both the United States and Singapore, it was found that as the price rose, users paid much more attention to system integrity, recovery and recycling. An industry tax on HFC-134a has had the same effect in Australia. It is possible that higher taxes or an absolute cap on availability could aid Europe in reducing HFC-134a emissions.
Regardless of the system ultimately chosen, service and maintenance procedures must change. Anderson noted recover/recycle compliance in the United States and Europe is well below 100 percent and every possible effort must be made to achieve compliance. It is still legal in many places to perform "gas and go" repairs without addressing the cause of leakage.
A well-designed enhanced HFC-134a system will require much work by the manufacturers and suppliers. It also will require a strong program to educate the public about the values of such a system and to create the demand for environmentally friendly air conditioning systems. Programs such as those developed by the Mobile Air Conditioning Society may be of value in the European market.
Introducing any new chemical system, such as HFC-152a or carbon dioxide, is not easy, as they are measurably different from their predecessors. It will be necessary to make decisions based on the environmental performance of the system and make that the single avenue of commerce. Engineering can mitigate the risk of the proposed systems, but either of those alternatives - if properly designed and handled - could be acceptable.
It will also be necessary to ensure that parts are available, service personnel are properly trained and cars with updated or alternative air conditioning systems are sold only into markets where parts and service are available. Problems with those systems should be solved quickly so that a company that's too ambitious and introduces a system prematurely doesn't tarnish the reputation of the technology.
The U.S. EPA is coordinating a global risk assessment with the cooperation of the European Commission, the Japanese Ministry of Environment, the Australian greenhouse office and the Canadian Environmental Ministry. Results will be communicated to the designers and public authorities that regulate safety, so the safety concerns of both carbon dioxide and flammable refrigerants can be accommodated.
Alternatives and Implications
Presentations covered both the science and practice of working with alternate refrigerants. Attendees were told of thermal properties of various gases and systems as well as hearing case studies of barriers overcome during vehicle conversion. The reports that follow are grouped by topic, and often include material from more than one presenter or company.
Improving Present HFC-134a Systems
The change from CFC-12 refrigerant to the present HFC-134a systems was costly but beneficial. On a scale of Ozone Depletion Potential (ODP), CFC-12 is rated at 1 while its replacement is rated 0. However, HFC-134a remains a greenhouse gas, although less severe than its predecessor. R134a is rated on a scale of Global Warming Potential at 1300. Compare that to a rating of 8500 for CFC-12. (A non-damaging gas is rated at zero.)
One of the quickest and least expensive ways to reduce the presence of this refrigerant in the atmosphere is to reduce system leakage. Enhanced is the term used for a vehicle system with today's components upgraded to low-loss specifications. Generally, enhancement will require different hoses to reduce refrigerant leakage, better O-ring seals, and improved compressor sealing. System leakage is a function of system pressure; doubling the pressure results in approximately four times the leakage. Additional loss management can be achieved through control of system pressure during various stages of operation. Component manufacturers are working on more efficient compressors as well as introducing sophisticated electronic controls for variable-displacement compressors. Further efficiencies may be gained through computer management of cabin inlet air, the use of a refrigerant oil separator in the system, and the installation of a heat exchanger on the suction line.
It was noted that better pressure management and improved compressor efficiency not only reduced direct losses from the system, but also provided improved fuel economy when the air conditioning system was in use. Based on research, it is believed that a properly designed enhanced HFC-134a system could reduce system losses by as much as 50 percent.
The numeric target for loss from an enhanced system in the United States may become the same as the proposed European standard - no more than 20 grams/year for single evaporator systems and 25 g/yr for dual evaporator systems. All presenters agreed on the need for a single, industrywide method of certifying mobile systems as low-loss.
Secondary Loop Systems
Secondary loop refrigeration systems are not new; they are in successful use in various industrial and fixed applications. The idea is relatively new within the automotive world.
In present mobile direct-expansion air conditioning systems, the refrigerant is supplied to the evaporator and heat exchange takes place at the evaporator. Heat is absorbed, cold air is passed to the cabin and the refrigerant begins its journey back to the compressor.
In a secondary loop system, the heat exchange to the refrigerant takes place in a separate heat exchanger under the hood. Instead of cooling the cabin air directly, the refrigerant cools a liquid coolant mixture - usually 50/50 water and anti-freeze - which is then circulated into the vehicle's heat exchangers in the cabin, providing cool air to the passenger compartment. Although still in development, these systems may offer several areas of improvement in climate management.
With the installation of additional valves, one heat exchanger might be used for both heating and cooling, thus eliminating a component under the dash. Because the refrigerant system is completely underhood, a smaller compressor using a smaller amount of refrigerant could still accomplish the same cooling effect. Additionally, testing shows that the system refrigerant charge need not increase if more than one heat exchanger is present. The coolant mixture goes to the in-car heat exchangers, not the refrigerant.
Other benefits include virtual elimination of heat exchanger icing problems as long as the outside air temperature is above freezing. Removing the expansion valve (or its equivalent) to the underhood position reduces in-car noise. And by keeping the refrigerant loop completely under the hood, cabin occupants have a greatly reduced risk of exposure to escaping refrigerants in case of a system rupture.
Secondary loop cooling systems allow distribution of the coolant mixture to additional evaporators or even to provide cooling for seats. The secondary loop system can use a variety of refrigerants since its reduced charge is limited in amount and contained in the engine compartment.
A secondary loop system will require additional fluid pumps, adding weight and complexity, as well as increasing the electrical load. Some questions remain regarding the most suitable refrigerant for the primary side, as well as questions about the best design and material for the interface between loops.
One alternate refrigerant under close examination is HFC-152a. This chemical is already in use as an aerosol propellant and is often the main ingredient in cans of spray cleaner for electronics and computer keyboards.
HFC-152a has a GWP of 130, about 94 percent less than HFC-134a (GWP 1300). Due to its very low GWP, it is not affected by the proposed European legislation and may be used after 2012. Compared to HFC-134a, it is attractive because of its better thermal efficiency. With improved thermal efficiency, a lower system charge may be used in a direct expansion system to achieve satisfactory cabin cooling. The amount of charge is further reduced if it is used in a secondary loop system.
This refrigerant is compatible with many existing components, so economies of conversion may be realized. On test vehicles, this refrigerant showed quicker initial cooling ("pull-down") and also lower compressor pressures on both suction and discharge lines. The reduced pressures allowed longer operation during hot weather idle before the compressor shut down.
The main drawback to the use of HFC-152a is that it is mildly flammable. In liquid form, it can be ignited with open flame and therefore may pose some hazard to vehicle occupants. If it is to be circulated into the cabin evaporator, a system of sensors and safety releases must be included to guard against unwanted discharge.
One such system presented at the symposium incorporates a sensor in the evaporator case and two relief valves in the refrigerant lines, one each for the high and low pressure lines. Detected leakage in the cabin or an over-pressure condition in the lines (which could force leaks in the system) cause small electric squibs to fire, puncturing diaphragms in the release valves. The squibs are similar to those used in seat belt tensioners or airbags. The evaporator sensor must see three consecutive readings above the programmed limit before it will cause a discharge.
According to the presenter, the system can be vented in just less than six seconds. The overboard discharge is ducted outside the passenger compartment. While it is easily possible to wire the trigger circuit into the airbag system, that option is still being evaluated.
Also under consideration for this system is a service or decommission switch that would disarm the protective system. This may be needed in shop environments where a concentration of HFC-152a may accumulate. A car sitting in this environment might expose the evaporator sensor to a level of refrigerant sufficient to cause an unwanted discharge.
Carbon Dioxide (CO2) Systems
Designated R744 when used as a refrigerant, CO2 occurs naturally in our atmosphere. Both its GWP and ODP ratings are zero, so it may be viewed as totally non-damaging to the environment. The substance is nonflammable and is used in some fire extinguishers. Early experiments using it as a refrigerant were limited by materials technology and the inability to contain the high operating pressures required to achieve satisfactory cooling. Many of these obstacles are now overcome and CO2 refrigeration systems are currently used in many rigid-pipe, fixed applications around the world.
As with any new system, there are considerations of cost vs. weight vs. efficiency vs. safety. One hallmark of a CO2 system is its higher operating pressures. Some demonstration systems run at pressures of 750-1200 psi in normal operation and easily exceed that under heavy thermal loads. Components must withstand not only these pressures but also incorporate a safety factor of four to five times the expected maximum. One presenter cited development of a hose with burst strength of approximately 9000 psi. Because of the difficulty of sealing a moving part, compressor seals need significant improvement before use in a CO2 system.
Several presenters made the point that use of this refrigerant will require new designs, new components and should not be made to emulate present systems in terms of layout and execution. These systems work best when designed from the ground up, instead of trying to convert an existing system. While CO2 shows promise as a refrigerant, manufacturers must overcome issues of cost, occupant safety, and efficiency to ensure its viability as a mobile A/C refrigerant in the world market.
Although current systems on test vehicles approximately equaled the performance of an HFC-134a system, development is ongoing to improve carbon dioxide technology. Improvements were achieved through use of small, highly efficient compressors as well as redesigned gas coolers (equivalent to condensers). Some compressors may be driven electrically; however, this requires that the vehicle have the ability to meet the electrical demands of the compressor.
Another potential benefit of a CO2 system is its ability to work as a heat pump, providing both cooled or heated air to the cabin. Research continues into developing the interface necessary to allow both heat and cooling.
Much development of carbon dioxide systems is taking place in Germany in a consortium of German manufacturers working toward standardized system components and unified specifications. They feel that established standards will reduce costs of manufacture. The target lifetime for such a system is proposed to be 300,000 km (180,000 miles) or 15 years. Some other proposed standards are:
Environmentalists continue to call for reduction of carbon dioxide emissions from industrial and other nonautomotive sources. Use of the gas as a refrigerant is seen by some as a net-zero enterprise, requiring no or very little new manufacture. CO2 is a byproduct of many industrial processes and can be easily recovered and purified. Some of what would normally be released to the atmosphere will be captured and channeled into refrigerant systems for vehicle use.
Safety and Fuel Efficiency
Two reports were presented on safety studies of alternate refrigerants; one study is completed and one is under way. A comparative analysis of potential risks of these products was recently performed by RISA of Berlin, Germany. RISA personnel's study considered HFC-152a, CO2, and R290 (propane). They found that in sufficient quantity, exposure to any of these gases - particularly in an enclosed area - might have potential harmful effects on humans. Various degrees of exposure to each refrigerant may cause symptoms of light-headedness, dizziness, disorientation or loss of concentration. Since mobile A/C systems can be operated in outside and recirculated air modes, occupant exposure is a concern and these results have implications for service technicians and storage facilities.
They also analyzed potential mechanical risks of new systems. Findings included the fact that careful location of components was important to avoid damaging adjacent items during a high-pressure failure.
Flammability studies found that, while it is an efficient refrigerant, R290 propane is extremely explosive and would present large risks to both cabin occupants and service personnel. HFC-152a was found only moderately flammable, but even reduced risk must be addressed before it comes into common use. The study found CO2 is an environmentally friendly refrigerant, but cautioned that the higher line pressures posed some risk to occupants, technicians and surrounding equipment.
EPA Risk Study
Erin Birgfeld of the U.S. Environmental Protection Agency explained her ongoing risk assessment study for alternate refrigerants. The study's intent is to form policy alternatives to HFC-134a refrigerants and will include evaluation of foreseeable "worst case" accident situations.
Vehicle Fuel Use
Emphasizing the need for increased fuel economy and reduced tailpipe emissions, a presentation by the National Renewable Energy Laboratory (NREL) offered some interesting statistics. According to their studies, approximately 5.5 percent of total motor vehicle fuel consumption in the United States is used for air conditioning. This equates to about seven billion gallons per year and this use alone contributes 62 billion kilograms of tailpipe CO2 to the atmosphere.
NREL further estimates that if the power required to drive the compressor could be reduced by 30 percent, fuel savings in the United States would be approximately 1.5 billion gallons per year and tailpipe emission of CO2 would drop by 22 billion kg. Numerically smaller but proportional savings would also be seen in Europe and Asia.
Based on the presentations, it is clear the next decade will bring many changes to the automotive climate-control industry. Some will be driven by regulatory legislation, some by industrywide voluntary agreements, and some by new materials and technologies.
Vehicle manufacturers will consider all the variables of any new system. Size, weight and cost are as important as occupant safety, function, and ease of use and repair. Component suppliers and system integrators must balance costs while providing a functional and efficient product. New systems will be tested against globally accepted standards, some yet to be developed.
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